Question

Use Molecular Orbital theory to show why cyclobutadiene is antiaromatic but the cyclopentadienyl anion is aromatic. (Hint: use the polygon rule to predict the molecular orbital energy diagrams).

Answer 1

For understanding the polygon rule draw the cyclobutadiene structure with one vertex facing the bottom of hte paper. Then, draw a horizontal line through the middle of the polygon (i.e cyclobutadiene). This line represents the energy level of the non-bonding orbital. Now draw the molecular orbitals by letting each vertex represents one molecular orbital. Now fill the electrons according to Hunde's rule. Since the two non bonding electrons will go into the opposite orbitals, this makes the cyclobutadiene a highly unstable diradical species. Hence cyclobutadiene is antiaromatic.

Draw a similar diagram for cyclopentadienyl anion and start filling the  electrons according to Hunde's rule. The six pi-electrons meets the criteria for Huckel's rule. Hence cyclopentadienyl anion is aromatic.